The present invention relates to a method of heat treating a component, in particular to a method of heat treating a turbine disc, a compressor disc, a turbine cover plate, a compressor drum or a compressor cone.
Nickel superalloy components, or articles, e.g. discs, for gas turbine engines, undergo a simple heat treatment after thermo-mechanical forming to the component, or article, shape e.g. disc shape. Normally this is a single stage isothermal solution heat treatment at a temperature either above (supersolvus) the gamma prime solvus (γ′) or below (subsolvus) the gamma prime solvus (γ′), followed by quenching in some medium, e.g. air or oil. The γ′ solvus is the critical temperature in alloys of this nature.
Solution heat treating below the γ′ solvus results in a fine grain microstructure, with a tri-modal distribution of the intermetallic strengthening phase, γ′, termed primary, secondary and tertiary. Solution heat treating above the γ′ solvus dissolves the primary γ′ present on the grain boundaries and allows the grains to coarsen to yield a coarse grain structure and bi-modal γ′ distribution, secondary and tertiary.
The solution heat treatment is then followed by a lower temperature age, or lower temperature ages, to relieve residual stresses that develop as a result of the quench and to refine the main strengthening precipitates for optimum mechanical properties. The single solution heat treatment temperature results in a component, e.g. a disc, with a uniform grain structure, either fine if a subsolvus solution heat treatment or coarse if a supersolvus solution heat treatment, and therefore a trade off in mechanical properties, performance, i.e. coarse grains for high temperature creep and fatigue crack growth resistance or fine grains for low temperature low cycle fatigue resistance and tensile strength.
It is known to provide a more complex heat treatment to a nickel superalloy component, e.g. a disc, this is dual-microstructure heat treatment, which results in a dual microstructure in the component, disc. The dual microstructure optimises the microstructure in different areas of the component, e.g. disc, based on the most important property for that area of the component in service, e.g. a fine grain structure in the hub, or bore, of the disc and a coarse grain structure in the rim of the disc. In this method the component is subject to a temperature gradient during the solution heat treatment. The rim of the disc is exposed to a temperature above the γ′ solvus while the hub, or bore, of the disc is maintained at a temperature below the γ′ solvus.
U.S. Pat. No. 6,610,110 discloses a method of heat treating a nickel superalloy disc comprising placing thermal bocks, heat sinks on the hub of the disc, enclosing the thermal blocks and the disc, except for the rim of the disc, within a shell and providing insulation within the shell, placing the assembly of disc, thermal blocks, shell and insulation in a furnace at a temperature above the gamma prime solvus temperature. The rim of the disc heats up at a faster rate than the insulated hub of the disc. The rim of the disc reaches a temperature above the gamma prime solvus temperature to coarsen the microstructure in the rim of the disc. A thermocouple is embedded in one of the thermal blocks and the assembly is removed when the thermocouple reaches a predetermined temperature. The disc has a diameter of 32 cm and an axial width of 5 cm at the hub and an axial width of 2.5 cm at the rim.
A problem with this method is that the discs used on larger gas turbine engines have much greater diameters and have much greater axial widths particularly at the hub of the disc. The greater size, and greater thermal mass, of the hub of these discs may result in the near surface regions of the hub reaching the equilibrium temperature, whilst the centre region of the hub reaching a much lower temperature, for example several hundred degrees centigrade lower. The centre region of the hub may be below the required subsolvus solution heat treatment temperature and in the ageing heat treatment regime. The effect of the hub of the disc obtaining a temperature significantly lower than the gamma prime solvus is to rapidly coarsen the gamma prime precipitates if the temperature is too low or to dissolve the gamma prime precipitates if the temperature is too high for ageing and too low for solution heat treatment. This would result in a disc with an overaged bore and a significant reduction in mechanical properties, thus negating the benefit of the dual microstructure heat treatment.